(a) Field of the Invention
The present invention relates to an objective for a binocular stereomicroscope and, more particularly, to an objective of so-called infinite distance correction type which is arranged that the rays which come out from the objective form an afocal image and which is to be used with a single-objective type binocular stereomicroscope (a binocular stereomicroscope which has only one objective).
(b) Description of the Prior Art
The perception of depth and distance is caused when observing a specimen through a stereomicroscope. The largest factor that causes the perception of depth and distance is the parallax between both eyes.
On the other hand, when observing a specimen having a flat surface through a stereomicroscope, the observed image sometimes does not look like a flat surface but looks like a convex surface. This phenomenon inevitably occurs more or less on the single-objective type binocular stereomicroscopes. The phenomenon that a flat surface looks like a convex surface is caused by the perception of depth and distance, i.e., it occurs because the marginal portion is perceived to be deeper and more distance, compared with the center of field, as the distance from the center of field becomes larger.
The fact to cause the perception of depth and distance is an important characteristic of stereomicroscopes which other types of microscopes do not have. The fact that an error is caused in the perception of depth and distance so that the flat surface of the specimen looks like a convex surface as mentioned in the above is a disadvantage of single-objective type stereomicroscopes.
The error in the perception of depth and distance is caused due to the fact that asymmetrical deformation of image, which does not occur at the time of observation through microscopes in general, occurs in the image observed through a single-objective type binocular stereomicroscope. As a result, a deviation occurs in the parallax between both eyes and causes the error in the perception of depth and distance. However, there is no known single-objective type binocular stereomicroscope arranged to correct the asymmetrical deformation of image by noticing the above-mentioned cause of the error in the perception of depth and distance.
For example, in case of the single-objective type stereomicroscope disclosed in Japanese published unexamined patent application No. 203411/83 (German Pat. No. 32 17 776), the embodiments thereof show that the value of ##EQU2## which shows the asymmetrical deformation of image as described later is large, i.e., 0.015 to 0.05. This value is large to the degree that, when observing a flat surface, the observer gets hallucinated that he is looking, for example, at a spherical surface of a radius about 400 mm to 120 mm in a field of a diameter of 100 mm formed at a distance of distinct vision of 250 mm.